bpo-36974: implement PEP 590 (GH-13185)

Co-authored-by: Jeroen Demeyer <J.Demeyer@UGent.be>
Co-authored-by: Mark Shannon <mark@hotpy.org>

Objects/classobject.c

@@ -40,6 +40,45 @@ PyMethod_Self(PyObject *im)
     return ((PyMethodObject *)im)->im_self;
 }
 
+
+static PyObject *
+method_vectorcall(PyObject *method, PyObject *const *args,
+                  size_t nargsf, PyObject *kwnames)
+{
+    assert(Py_TYPE(method) == &PyMethod_Type);
+    PyObject *self, *func, *result;
+    self = PyMethod_GET_SELF(method);
+    func = PyMethod_GET_FUNCTION(method);
+    Py_ssize_t nargs = PyVectorcall_NARGS(nargsf);
+
+    if (nargsf & PY_VECTORCALL_ARGUMENTS_OFFSET) {
+        /* PY_VECTORCALL_ARGUMENTS_OFFSET is set, so we are allowed to mutate the vector */
+        PyObject **newargs = (PyObject**)args - 1;
+        nargs += 1;
+        PyObject *tmp = newargs[0];
+        newargs[0] = self;
+        result = _PyObject_Vectorcall(func, newargs, nargs, kwnames);
+        newargs[0] = tmp;
+    }
+    else {
+        Py_ssize_t nkwargs = (kwnames == NULL) ? 0 : PyTuple_GET_SIZE(kwnames);
+        PyObject **newargs;
+        Py_ssize_t totalargs = nargs + nkwargs;
+        newargs = PyMem_Malloc((totalargs+1) * sizeof(PyObject *));
+        if (newargs == NULL) {
+            PyErr_NoMemory();
+            return NULL;
+        }
+        /* use borrowed references */
+        newargs[0] = self;
+        memcpy(newargs + 1, args, totalargs * sizeof(PyObject *));
+        result = _PyObject_Vectorcall(func, newargs, nargs+1, kwnames);
+        PyMem_Free(newargs);
+    }
+    return result;
+}
+
+
 /* Method objects are used for bound instance methods returned by
    instancename.methodname. ClassName.methodname returns an ordinary
    function.
@@ -69,6 +108,7 @@ PyMethod_New(PyObject *func, PyObject *self)
     im->im_func = func;
     Py_XINCREF(self);
     im->im_self = self;
+    im->vectorcall = method_vectorcall;
     _PyObject_GC_TRACK(im);
     return (PyObject *)im;
 }
@@ -309,7 +349,7 @@ PyTypeObject PyMethod_Type = {
     sizeof(PyMethodObject),
     0,
     (destructor)method_dealloc,                 /* tp_dealloc */
-    0,                                          /* tp_print */
+    offsetof(PyMethodObject, vectorcall),       /* tp_vectorcall_offset */
     0,                                          /* tp_getattr */
     0,                                          /* tp_setattr */
     0,                                          /* tp_reserved */
@@ -323,7 +363,8 @@ PyTypeObject PyMethod_Type = {
     method_getattro,                            /* tp_getattro */
     PyObject_GenericSetAttr,                    /* tp_setattro */
     0,                                          /* tp_as_buffer */
-    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC, /* tp_flags */
+    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC |
+    _Py_TPFLAGS_HAVE_VECTORCALL,                /* tp_flags */
     method_doc,                                 /* tp_doc */
     (traverseproc)method_traverse,              /* tp_traverse */
     0,                                          /* tp_clear */